Liquid crystal display devices have been used for watches and electronic calculators, various measuring apparatuses, automotive panels, word processors, electronic notebooks, printers, computers, televisions, watches, advertising displays, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a VA (vertical alignment) mode and an IPS (in-plane switching) mode using TFT (thin-film transistor), and the like. Liquid crystal compositions used for these liquid crystal display devices are required to have stability to external factors such as moisture, air, heat, light, and the like, exhibit a liquid crystal phase within as wide a temperature range as possible including room temperature as a center, and have low viscosity and low drive voltage. Further, each of the liquid crystal compositions is composed of several types to several tens types of compounds in order to have optimum values of dielectric anisotropy (Δ∈) and/or refractive index anisotropy (Δn) for a display device.
A vertical alignment-mode display uses a liquid crystal composition having negative Δ∈ and are widely used for liquid crystal TV and the like. On the other hand, low-voltage driving, fast response, and a wide operating temperature range are required for all driving methods. That is, a large absolute value of positive Δ∈, low viscosity (η), and a high nematic-isotropic liquid phase transition temperature (Tni) are required. Also, in view of setting of Δn×d which is the product of Δn and a cell gap (d), it is necessary to adjust Δn of a liquid crystal composition within a proper range according to the cell gap. In addition, when a liquid crystal display device is applied to a television or the like, fast response is regarded as important, and thus a liquid crystal composition having low γ1 is required.
On the other hand, a practical liquid crystal composition includes several types to several tens types of liquid crystal compounds, and the physical property values are determined by the compounds selected and the contents thereof. Many liquid crystal compounds have already been studied, and the basic physical property values such as liquid crystallinity, birefringence, dielectric anisotropy, and the like of the compounds have been made known, and also the basic physical property values of liquid crystal compositions have been mostly made known. However, with expanding application of liquid crystal display devices, significant changes are found in use methods and manufacturing methods thereof. In order to cope with these changes, it is demanded to optimize characteristics other than basic physical property values which have been known. That is, VA (vertical alignment)-mode and IPS (in-plane switching)-mode liquid crystal display devices using liquid crystal compositions are widely used, and supersized liquid crystal devices of 50 inches or more are put into practical application and used. With increases in substrate size, instead of a usual vacuum injection method, a one drop fill (ODF) method becomes the mainstream of a method of injecting a liquid crystal composition into a substrate (refer to Patent Literature 1), but when a liquid crystal composition is dropped on a substrate, the problem of degrading display quality by dropping marks is surfaced. Further, for the purpose of achieving fast response by forming pre-tilt angles of liquid crystal materials in liquid crystal display devices, a PS liquid crystal display device (polymer stabilized) and a PSA liquid crystal display device (polymer sustained alignment) have been developed (refer to Patent Literature 2), leaving the above-described problem as a great problem. That is, these liquid crystal display devices are characterized in that a monomer is added to a liquid crystal composition, and the monomer in the composition is cured. Compounds which can be used in liquid crystal compositions for active matrix are specified due to the need to maintain a high voltage holding ratio, and the use of compounds having an ester bond therein is limited. Monomers used in a PAS liquid crystal display device are mainly acrylate-based monomers and are generally compounds having an ester bond therein, and such compounds are generally not used as liquid crystal compounds for active matrix (refer to Patent Literature 3). Such foreign materials induce the occurrence of dropping marks and have the problem of degrading the yield of liquid crystal display devices due to display defects. In addition, when additives such as an antioxidant, a light absorber, and the like are added to a liquid crystal composition, deterioration in yield becomes a problem.
Here, “dropping marks” is defined as a phenomenon that white marks of dropping of a liquid crystal composition appear on the surface of a black display.
There is disclosed a method for suppressing dropping marks, in which a polymerizable compound mixed in a liquid crystal composition is polymerized to form a polymer layer in a liquid crystal layer, thereby suppressing dropping marks occurring in relation to an alignment control film (Patent Literature 3). However, this method has the problem of causing image sticking in a display due to the polymerizable compound added to a liquid crystal, and the effect of suppressing dropping marks is unsatisfactory. Therefore, development of a liquid crystal display device causing little image sticking and dropping marks while maintaining the basic characteristics as a liquid crystal display device has been demanded.